fabric FSDP strategy save/load checkpoint does not support s3 url

Nobody has claimed this yet.

### Bug description

Other ParallelStrategy implementations support saving and loading checkpoints from an S3 URL using fsspec. Fabric’s FSDP strategy also uses torch.load and torch.save, which support S3 URLs as well. However, the FSDP wrapper in Fabric [converts the input path](https://github.com/Lightning-AI/pytorch-lightning/blob/0c9d4147e3569e24bf6130e78ae21b136562766e/src/lightning/fabric/strategies/fsdp.py#L440) to a pathlib.Path object. This conversion changes a valid URL like s3://bucket/xxx into an invalid format like s3:/bucket/xxx, effectively breaking the path.

To resolve this issue, path manipulation should be handled using os.path instead—[as done in TorchCheckpointIO](https://github.com/Lightning-AI/pytorch-lightning/blob/0c9d4147e3569e24bf6130e78ae21b136562766e/src/lightning/fabric/plugins/io/torch_io.py#L57).

### What version are you seeing the problem on?

v2.5

### How to reproduce the bug

```python
- Use FSDP as the strategy for fabric.
- Save a checkpoint with `fabric.save("s3://xxx")`
```

### Error messages and logs

```
# Error messages and logs here please
```

### Environment

_No response_

### More info

_No response_

cc @lantiga @justusschock

Start in src/lightning/fabric/strategies/fsdp.py at the checkpoint path conversion around line 440, then compare the path handling in src/lightning/fabric/plugins/io/torch_io.py around line 57. Reproduce the issue with FSDP and fabric.save("s3://xxx"); done means S3 URLs remain valid for checkpoint save and load.

Written by the indexing model from the issue text.